Frequently, broadband systems transmit television signals to subscribers of a conditional access system. Broadband systems, such as cable and satellite television systems, typically include a headend for receiving programming, or sessions, and/or data from various sources and redistributing the programming and other data through a distribution system to subscribers. The headend receives programming signals from a variety of sources, combines the programming signals from the various sources, and transmits the combined signals through the distribution system to subscriber equipment. The distribution system can include a variety of media, such as coaxial cable, fiber optic cable, and satellite links. In a subscriber television system, the subscriber equipment, which receives the signals from the headend, can include a cable-ready television, a cable-ready video cassette recorder (VCR), or a digital home communications terminal (DHCT) that is connected to a television, computer, or other display device.
The headend uses modulators to control the streams of data into the distribution system. In today's competitive market, the modulators must be able to accept data/programming from equipment manufactured by many different suppliers. Increasingly, the headend is receiving and transmitting programming in a digital format, for example, Moving Pictures Expert Group (MPEG) format, instead of an analog format. Transmitting programs in MPEG format is advantageous because multiple digitized programs can be combined and transmitted in, for example, 6 MHz of bandwidth, which is the same amount of bandwidth that is required to transmit a single analog channel or program.
MPEG transport streams include overhead information such as MPEG tables that indicate the types and location of the programming within the transport stream. In a local television system, the MPEG tables include information that is specific to that local distribution system and its particular channel line-up. MPEG as referenced in this application is described in the MPEG-1 and MPEG-2 standards. The MPEG-1 standards (ISO/IEC 11172) and the MPEG-2 standards (ISO/IEC 13818) are described in detail in the International Organization for Standardization document ISO/IEC JTC1/SC29/WG11 N (June 1996 for MPEG-1 and July 1996 for MPEG-2), which is hereby incorporated by reference. Therefore, the headend system, and the modulators in particular, must insure that the required MPEG table data is contained in the outgoing bit stream.
Content and data providers provide streams of data, data streams, that include video, audio and data, to system operators via video sources, such as video encoders and video servers. The data streams are initially prepared for transmission through the broadband system by programming, or mapping, the video, audio and data with control software within a digital network control system (DNCS), which is an element manager for processing data within the headend. The DNCS causes the data streams associated with several programs to be combined into bundled groups of sessions. More specifically, the system operator defines and maps the specifications of the individual data streams from one or several content and data providers and, for example, multiplexes them into grouped sessions in order to maximize the use of the bandwidth available within the subscriber television system.
In any broadband system there is a limited amount of bandwidth available. For example, a typical subscriber television system has a forward bandwidth of 50 Megahertz (MHz) to 870 MHz, which is divided into channels. Therefore, a limited number of modulated channels can be delivered to a particular DHCT. An example of a modulator is a quadrature amplitude modulation (QAM) modulator that receives a digital bit stream and modulates it for transmission over the subscriber television network. Typically, a channel occupies 6 MHz of bandwidth, and a QAM modulator can generally modulate and transmit data through the bandwidth at a rate of approximately 27 or 38 Mega bits per second depending upon the mode of QAM modulation used. The modulator modulates the bundled group of sessions with a particular radio frequency (RF) and the modulated signal is provided to the output port of the modulator. A combiner then combines the modulated sessions with other outputs from modulators. The combined modulated outputs are then provided downstream via a distribution network to a plurality of DHCTs. There are numerous bundled groups of sessions that can be programmed by the DNCS and provided to numerous modulators; however, each bundled group is modulated with a different frequency across all the modulators.
In a typical broadband subscriber television system, the efficiency of the system is optimized by choosing a bundled group of sessions such that the bit rate of the bundled group of session is close to, but does not exceed, the bandwidth limitation of the modulator. Typically, a bundled group of sessions includes a program or data information that has a variable bit rate, and consequently the bit rate of the bundled group of sessions cannot be precisely predetermined. Care must be taken by the operator of the system to make ensure that the bit rate for the bundled group of sessions does not exceed the bandwidth limitation of the modulator. Otherwise, a condition, which is known to those skilled in the art as macro-blocking, occurs when information from the bundled group of sessions is not transmitted from the modulator because the bit rate of the bundled group of sessions exceeds the bandwidth limitation of the modulator. One method for eliminating macro-blocking is to reduce the bit rate of the bundled group of sessions by reducing the number of sessions in the bundled group of session. Another method is to statistically multiplex the group of sessions. However, some or all of the programs or data information provided by the content providers to the subscriber television system is encrypted, and encrypted content cannot be statistically multiplexed.